Field of the Invention
The present invention relates to a sensor control apparatus for controlling a gas sensor which includes a detection cell for producing an electromotive force corresponding to a particular component contained in an object gas and an oxygen pump cell for performing pumping in or pumping out of oxygen in accordance with pump current, and to a gas detection system which includes the gas sensor and the sensor control apparatus.
Description of Related Art
There have been known a sensor control apparatus for controlling a gas sensor having a detection cell and an oxygen pump cell and a gas detection system which includes the gas sensor and the sensor control apparatus.
The detection cell of the gas sensor is a cell which produces an electromotive force corresponding to a particular component contained in an object gas, and the oxygen pump cell of the gas sensor is a cell which performs pumping in or pumping out of oxygen in accordance with pump current.
An example of such a gas sensor is an A/F sensor (oxygen sensor) which pumps in oxygen or pumps out oxygen contained in an object gas through use of an oxygen pump cell such that the electromotive force produced in a detection cell becomes equal to a target voltage set in advance and which linearly detects the concentration of oxygen contained in the object gas on the basis of the pump current flowing (supplied) to the oxygen pump cell at that time. Another example of such a gas sensor having an oxygen pump cell is an NOx sensor for detecting the concentration of NOx contained in an object gas.
Such a sensor control apparatus must have various computation functions for controlling the pump current of such a gas sensor, and must meet the demand for miniaturization of the apparatus. In view of these demands, a known sensor control apparatus employs a digital control section in place of an analog circuit for performing various controls (Patent Document 1).
Such a sensor control apparatus employing a digital control section includes analog-to-digital conversion sections (AD converters) which convert various analog signals detected from a gas sensor (for example, electromotive force produced in the detection cell) to digital values.
Another known sensor control apparatus is configured to switch a plurality of analog signals using a multiplexer to thereby allow conversion of analog signals to digital values even when the number of AD converters is smaller than the number of the analog signals.
In such a sensor control apparatus, the pump current of a gas sensor can be controlled by performing digital PID control while using various digital values converted from various analog signals (e.g., electromotive force produced in the detection cell of the gas sensor).
The above-mentioned digital control section can be rendered smaller in size as compared with an analog circuit. Also, when such a digital control section is used, an operation of changing control constants becomes easier as compared with the case where an analog circuit is used. Therefore, it becomes easier to adapt to control of gas sensors of a larger number of types and control of gas sensors having various characteristics.